Searches for rare Bs Decays with the DØ Detector Ralf Bernhard University of Zürich HEP Seminar University of Freiburg May 10th 2006

Outline Motivation for FCNC searches FNAL and DØ Detector Search for the Decay Bs → μ+μ- Search for the Decay Bs →  μ+μ- Observation of the Decay Bs →  ψ(2S) Summary

History of FCNC Particle physics until the '70 knew only three light quarks (u,d,s) which could mix due to the Cabibbo angle θc As a consequence s → d transitions can occur because This was in contradiction with experimental situation in '64 – '70, no such transition in Kaon decays were observed (limits order of 10-6) In 1970 Glashow, Iliopoulos, Maiani (GIM) proposed a new quark (charm) to cancel the unobserved FCNC transitions (at tree level). Historically the GIM mechanism allowed charm mass prediction before it was observed in J/ψ (cc) resonances 1974 d s  Z

Purely leptonic B decay B->l+ l- decay is helicity suppressed FCNC SM: BR(Bs->m+m-) ~ 3.410-9 depends only on one SM operator in effective Hamiltonian, hadronic uncertainties small Bd relative to Bs suppressed by |Vtd/Vts|2 ~ 0.04 if no additional sources of flavor violation reaching SM sensitivity: present limit for Bs -> m+m- comes closest to SM value SM expectations: Current published limits: Br(Bdl+l-) Br(Bsl+l-) l = e 3.4 × 10-15 8.0 × 10-14 l=μ 1.0 × 10-10 3.4 × 10-9 l=τ 3.1 × 10-8 7.4 × 10-7 C.L. 90% Br(Bdl+l-) Br(Bsl+l-) l = e < 6.1 ·10-8 < 5.4 ·10-5 l=μ < 8.3 ·10-8 <1.5 x 10-7 l=τ < 3.1·10-3 < 5.0%

Purely leptonic B decay excellent probe for many new physics models particularly sensitive to models w/ extended Higgs sector BR grows ~tan6b in MSSM 2HDM models ~ tan4b mSUGRA: BR enhancement correlated with shift of (g-2)m also, testing ground for minimal SO(10) GUT models Rp violating models, contributions at tree level (neutralino) dark matter … Two-Higgs Doublet models: Rp violating:

Motivation for FCNC searches FNAL and DØ Detector Search for the Decay Bs → μ+μ- Search for the Decay Bs →  μ+μ- Observation of the Decay Bs →  ψ(2S) Summary

TeVatron New Main Injector and Antiproton Reycler Increase number of bunches 6×6→ 36× 36 Reduce bunch spacing 3.5μs → 396ns Increase beam energy 900 GeV → 980 GeV Projected integrated luminosity per experiment: ≈ 2 fb-1 2006 ≈ 8 fb-1 2009 Highest initial luminosity so far 1.7×1032 cm-2 s-1 1.18fb-1 recorded per experiment Data taking efficiency: 85-90%

Integrated Luminosity

B production at the TeVatron bb cross section orders of magnitude larger than at B-factories (4S) or Z σ(pp → bb) = 150μb at 2TeV σ(e+e- → Z → bb) = 7nb σ(e+e- → Υ(4S) → bb) = 1nb all kinds of b hadrons produced: Bd, Bs, Bc, B**, b, b, … However: QCD background overwhelming, b-hadrons hidden in 103 larger background events complicated, efficient trigger and reliable tracking necessary crucial for B physics program: good vertexing & tracking triggers w/ large bandwidth, strong background rejection muon system w/ good coverage e.g., integrated cross sections for |y|<1: (B+, pT  6 GeV/c)~4 mb Lots going on in Si detector

The DØ Experiment Excellent coverage of Tracking and Muon Systems Forward muon system with |η|<2 and good shielding 4-layer Silicon and 16-layer Fiber Trackers in 2 T magnetic field SMT

Tracking System small tracking volume w/ radius ~0.5 m impact parameter resolution: ~50 m at pT ~ 1 GeV/c ~10 m at higher pT 2nd vertex resolution ~40 m (r,) ~80 m (r, z)

Muon System 3 layer of drift tube + scintillators ( < 2) Toroid magnet between 1st and 2nd layer allows stand-alone momentum measurement Central Proportional Drift Tubes 6624 drift cells (10.1 cm  5.5 cm) Stacked in 3- and 4- deck chambers Forward Mini Drift Tubes 6080 8-cell tubes (9.4mm  9.4 mm) Provides fast L1 trigger signal Scintillation Counters (forward and central) 4214 forward, 630 central counters Segmentation 0.1mm * 4.5mm in  Provide fast L1 trigger signal

Triggers for B physics robust and quiet di-muon and single-muon triggers keys to B physics program at DØ large coverage ||<2, p>1.5-5 GeV – depends on Luminosity and trigger variety of triggers based on Level 1/2: based on Muon hits aided by Fiber Tracker (hardware/hybrid) Level 3: flexible and fast reconstruction of full event typical total rates at medium luminosity (7 x 1031 s-1cm-2) di-muons : 75 Hz / 20 Hz / 2 Hz @ L1/L2/L3 single muons : 120 Hz / 100 Hz / 50 Hz @ L1/L2/L3 (has to be prescaled) muon purity @ L1: 90% - all physics! Current total trigger bandwidth (input ~1.6 MHz) 1800 Hz / 800 Hz / 50 Hz @ L1/L2/L3

Motivation for FCNC searches FNAL and DØ Detector Search for the Decay Bs → μ+μ- Search for the Decay Bs →  μ+μ- Observation of the Decay Bs →  ψ(2S) Summary

Di-Muon Data Sample Signal Region (not able to separate Bs and Bd) 300 pb-1 Signal Region (not able to separate Bs and Bd)

Strategy Published a limit using 240pb-1 Using a slightly larger data set (300pb-1) for a Tevatron combination note (which yielded the current best published limit) Using additional recorded data Obtain sensitivity (blind analysis) with additional data set w/o changing the analysis procedure Combine sensitivity with existing published limit Used in previous analysis Still blind!

Selection Cuts Cut on Mass region of di-muon sample 4.5 < m < 7 GeV/c2 Two good muons with a net charge of zero and a pT greater than 2.5 GeV The triggered muons have reconstructed tracks in the tracker with at least 3 hits in the Silicon tracker at least 4 hits in the Fiber tracker Good reconstructed vertex Cut on the uncertainty of the transverse decay length (Lxy) < 150 m A minimum pT of the Bs candidate of 5 GeV is required 38k events remain 300 pb-1 blinded signal region: 5.160 < m < 5.520 GeV/c2; ±2 wide, =90 MeV Sideband regions: 540 MeV/c2 each

Searching the Needle! Using discriminating variables! Potential sources of background: continuum  Drell-Yan sequential semi-leptonic b->c->s decays double semi-leptonic bb-> X b/c->x+fake fake + fake Using discriminating variables!

Discriminating Variables Opening angle between the vertex direction and the muon pair "Pointing consistency" Decay length significance (Lxy /σ(Lxy)) Isolation of the B candidate with

Optimisation Procedure Optimise cuts on a data sub sample data and keep signal region as blind box Performed random grid search of the 3 discriminating variables Maximise sensitivity of searches for new signals (physics/030863) Define α as significance of the test a is the number of sigmas for α (i.e 95% →2σ→a = 2)

Optimization Procedure II Correct statistical practice requires to decide before the experiment the values of  and CL S/√B may push the experiment efficiency down to very small values, e.g. 0.1 expected signal events with a background of 10-5 over 10 signal expect and 1background event S/√(S+B) cannot be maximized without knowing the x-section of the searched signal Independent of the expectations for a signal to be present thus allowing an unbiased optimization No dependence on metric or priors Independent of choice of a limit setting algorithm Punzi’s proposal can be for setting limits and discovery, by setting the constant a

Optimisation Results Decay length significance: > 18.5 Opening angle: α < 0.2 rad Isolation: Iso > 0.56 Expect 4.3 ± 1.2 background events Observe 4 events in Signal Region

Normalisation Channel B+→J/ψK+ Use the decay of the decay J/ψ →μ+μ- to cancel μ+μ- efficiencies Vertex an additional track to the di-muon pair Additional cuts on the Kaon and B candidate are: Kaon pT > 0.9 GeV/c Collinearity of > 0.9 is required χ2 of the vertex fit contribution not more than 10, together not more than 20 Fit of a Gaussian as signal plus a quadratic function as background.

DØ Sensitivity 400 pb-1 additional Cut Values changed only slightly! Expect 2.2 ± 0.7 background events additional 400 pb-1

Limit Calculation R = BR(Bd)/BR(Bs) is small due to |Vtd/Vts|^2 eB+ /eBs relative efficiency of normalization to signal channel eBd /eBs relative efficiency for Bd-> m+ m- versus Bs-> m+ m- events in Bs search channel (~0.95) fs/fu fragmentation ratio (in case of Bs limit) - use world average with 15% uncertainty DØ Bs->mm 240 pb-1 5.1×10-7 Published 300 pb-1 4.0×10-7 Prelim. DØ <Bs->mm 700 pb-1 <2.3×10-7> Sensitivity all limits below are 95% C.L. Bayesian incl. sys. uncertainty

Systematic Uncertainties Efficiency ratio determined from MC with checks in data on trigger/tracking etc. Large uncertainty due to fragmentation ratio Background uncertainty from interpolating fit

Tevatron limit combination I fragmentation ratio b->Bs/b->Bu,d standard PDG value as default Tevatron only fragmentation (from CDF) improves limit by 15% uncorrelated uncertainties: uncertainty on eff. ratio uncertainty on background correlated uncertainties: BR of B± -> J/(->mm) K± fragmentation ratio b->Bs/b->Bu,d quote also an average expected upper limit and single event sensitivity DØ has larger acceptance due to better h coverage, CDF has greater sensitivity due to lower background expectations

world-best limit, only factor 35 away from SM Combination II 2-Higgs Doublet Model world-best limit, only factor 35 away from SM BR(Bs-> m+ m- ) < 1.2 (1.5) × 10-7 @ 90% (95%) C.L. Combined TeVatron Limit: R. Bernhard et al. hep-ex/0508058

Constraining dark matter mSUGRA model: strong correlation between BR(Bs->m+m-) with neutralino dark matter cross section especially for large tanb constrain neutralino cross section with less than, within and greater than 2 of WMAP relic density universal Higgs mass parameters CDF & DØ CDMS non-universal Higgs mass Parameters, dHu=1, dHd=-1 CDF & DØ S. Baek et al., JHEP 0502 (2005) 067

Ms vs Bs  +-

Prospects Expectation for Bs → μ+μ- DØ TeVatron

Motivation for FCNC searches FNAL and DØ Detector Search for the Decay Bs → μ+μ- Search for the Decay Bs →  μ+μ- Observation of the Decay Bs →  ψ(2S) Summary

Search for Bs ->  m+m- long-term goal: investigate b -> s l+ l- FCNC transitions in Bs meson exclusive decay: Bs ->  m+m- SM prediction: short distance BR: ~1.6×10-6 about 30% uncertainty due to B-> form factor 2HDM: enhancement possible, depending on parameters for tanb and MH+ presently only one published limit CDF Run I: 6.7×10-5 @ 95% C.L.

Search for Bs ->  m+m- Dilepton mass spectrum in b -> s l l decay 300 pb-1 of dimuon data normalize to resonant decay Bs -> J/y f cut on mass region 0.5 < M(mm) < 4.4 GeV/c2 excluding J/y & y’ two good muons, pt > 2.5 GeV/c two additional oppositely charged tracks pt>0.5 GeV/c for f f candidate in mass range 1.008 < M(f) < 1.032 GeV/c2 good vertex pt(Bs cand.) > 5 GeV/c non-resonant decay: cut out J/ and ’ J/y Y(2S)

Search for Bs ->  m+m- Blind analysis: optimization with following variables in random grid search Pointing angle Decay length significance Isolation Background modeled from sidebands Use resonant decay Bs -> J/y f with same cuts as normalization Gaussian fit with quadratic background: 73 ± 10 ± 4 Bs-> J/y f resonant decays

Discriminating Variables Decay length significance: > 10.3 Opening angle: α < 0.1 rad Isolation: Iso > 0.72

Limit on Bs ->  m+ m- submited to PRL hep-ex/0604015 expected background from sidebands: 1.6 ± 0.4 events observe zero events in signal region BR(Bs -> f m+m-)/BR(Bs -> J/y f) < 4.4 × 10-3 @ 95% C.L. Using central value for BR(Bs -> J/y f) = 9.3×10-4 PDG2004: BR(Bs -> f m+m-) < 4.1×10-6 @ 95% C.L. x10 improvement w.r.t previous limit submited to PRL hep-ex/0604015

Expected limit Bs ->  m+m- expected limit at 95% C.L. for Bs ->  m+m-

Motivation for FCNC searches FNAL and DØ Detector Search for the Decay Bs → μ+μ- Search for the Decay Bs →  μ+μ- Observation of the Decay Bs →  ψ(2S) Summary

Motivation for Bs →  ψ(2S) PDG says decay has been “seen” (1 event observed at ALEPH in 1992 when they measured the Bs mass ) Historically: The decay B+  (2S) K+ was observed at ARGUS 1990 B  (2S) K*0 was observed in CDF Run I in 1998 B  (2S) Ks and B+  (2S)K*+ by CLEO in 2000 Measurements show that the rates of B+ and B0 mesons decay to (2S) states is approximately 60% of the analogues decay to J/ The relative branching ratio Bs  (2S) / Bs  J/ was now recently measured by CDF (they published before us) Strategy: Use B+  (J/,(2S)) K+ as control channel Reconstruct the decay Bs  J/ Move the Di-Muon mass window to the (2S) resonance (3.45 GeV/c2 < m< 3.95 GeV/c2)

Control Channel Comparison with BaBar

Bs →  ψ(2S) Candidates Significance of 6σ Loose selection of candidates Use of Discriminating Variables Significance of 6σ Expect 1.8 ±1.3 events

Calculation of the Ratio

Summary A search for the FCNC decay Bs → μ+μ- has been presented. Importance of this decay to constrain models beyond the SM. We have more data recorded to further improve (observe) the limit (decay). Expect an update/combination for the summer. A search for the FCNC decay Bs → +μ- has been presented. The obtained limit improves the published limit by a factor of 10 (with just a 1/3 of the recorded data). This decay mode should be observable in Run II. The observation of decay Bs → (2S) has been presented. The results for the BR are in agreement with the expectations (around 60% with respect to the corresponding J/  mode).

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Summary A new expected sensitivity on the decay Bs → μ+μ- has been presented. Goal is to improve the sensitivity, using new discriminating variables and multivariate techniques, unblind if sensitive is around 1 10-7. A Limit on the decay Bs → μ+μ- has been presented. Improving the current published value by a factor of 10.

Example: SO(10) symmetry breaking model Implications Example: SO(10) symmetry breaking model R. Dermisek et al. hep-ph/0507233 Contours of constant Br(Bsμ+μ-)